PURPOSE: To regulate the retreat limit and the valve opening position by a bearing by forming a stopper part and a contact part which regulate the retreat limit through the mutual contact of the opposed surfaces of a bearing and a piston and opening a poppet valve by directly receiving a valve opening rod by a stopper part at the retreat limit of the piston.

CONSTITUTION: An annular projection 25 as stopper part and a shoulder part 26 as contact part are formed on the opposed surfaces of a bearing 7 and a rear part piston 22, and the retreat limit of the gear part piston 22 is regulated by the contact. Further, a valve opening rod 202 for pushing and separating the valve rod 232 of a poppet valve 18 is constituted so as to directly contact the projection 25, at both the edge parts at the retreat limit of the piston 22. Accordingly, the piston 22 is received by the bearing 7 at the retreat limit, and the poppet valve 182 is put into an opened valve state by the valve opening rod 202 at the retreat limit of the piston 22. Accordingly, the need of installing a stopper ring separatedly is obviated, and the quantity of the constitution ports can be reduced, and the length in the axial direction can be shortened.